The Atomic Force Microscopy Essay

The three major components of an AFM are piezoelectric transducers (in AFM, often known as piezoelectric scanners), force transducers (force sensors), and feedback control system. The basic principle of AFM as shown in Figure 9.4 is, piezoelectric transducer moves the tip over the sample surface, the force transducer senses the force between the tip and the surface, and the feedback control feeds the signal from the force transducer back in to the piezoelectric, to maintain a fixed force between the tip and the sample [9.14]. Piezoelectric materials are electromechanical transducers that convert electrical potential into mechanical motion and are used in controlling the motion of the probe while scanning the sample surface. The force transducer in an AFM is a cantilever with integrated tip (the probe), and an optical lever. AFM is more sensitive because the control electronics take the signal from the force transducers, and use it to drive the piezoelectrics so as to maintain the probesample distance, and thus the interaction force at a set level. Thus, if the probe registers an increase in force (for instance, while scanning the tip encounters a particle on the surface), the feedback control causes the piezoelectrics to move the probe away from the surface. Conversely, if the force transducer registers a decrease in force, the probe is moved towards the surface.
A variety of probes have been used but the most commonly used are microfabricated silicon (Si) or silicon nitride (Si3N4) cantilevers with integrated tips. The tips can be very sharp. Typically, probe radius varies from 5 to 20 nm. The bending of the cantilever, normal to the sample surface is usually monitored by an optical lever. The optical lever consists of a laser focused on the back of the cantilever, which is reflected onto a photodetector (typically a split photodiode). There are a variety of modes of scanning, but in the simplest mode, the probes touches gently the sample as it moves over the surface. The movement of the probe over the surface is controlled by the piezoelectric scanners which can move the probe very precisely in the x, y, and z axes. Therefore, if the scanning cantilever encounters a surface change in specimen, it must move specimen up/down with z portion of the scanner so that cantilever doesn't deflect. This type of data (not allowing cantilever to be deflected and using the voltage change of z portion of the piezo scanner) is referred as height mode [9.15]. The deflection causes the laser to change the position on the photodiode array, changing the output of photodiode array. The voltage change is detected by the feedback controller circuit, and thus controller sends the response to z-piezoelectric scanner to set back the voltage to original value, in order to maintain the probe-sample distance at a set value. Because the cantilever acts as a spring, this fixed cantilever deflection means a fixed probe-sample force is maintained. The response that the controller send to z- piezoelectric is used to construct the image of the specimen.